Did Mars's Magnetic Field Die With a Whimper or a Bang?

Giant asteroids may have wiped out Mars's magnetic field. The energy released by massive collisions upset the heat flow in the planet's iron core that produced the magnetism, according to a new study. The finding offers a solution to the mystery of the disappearing magnetic field and sheds light on early Earth conditions.

A planet's magnetic field results from a process called convection. Within the core, molten iron rises, cools, and sinks. The convection induces a magnetic field, in a system known as a dynamo.

Like Earth, early Mars had a magnetic field and perhaps an atmosphere conducive to liquid water. But magnetic analysis of the martian surface indicates that when Mars was a mere 500 million years old, its magnetic field withered away. Without this shield, streams of ionizing particles spewing from the sun strip away a planet's atmosphere, killing any life that may have emerged or forcing it underground.

The disappearance of the martian magnetic dynamo has puzzled scientists. One theory links it to the Late Heavy Bombardment, a period of 100 million years when asteroids--some hundreds of kilometers across--smashed into Mars and the inner planets. A massive collision could warm Mars's mantle, disrupting core convection. That's because the cooling action of the mantle draws heat from the core, keeping it churning. Without that flow, core convection grinds to a halt.

The theory fits with the observation that only the oldest impact craters on Mars are magnetized. Still, all remained speculation until data came back from the Mars Global Surveyor and other recent spacecraft. Last year, planetary scientists Robert Lillis and Michael Manga, both of the University of California, Berkeley, linked age estimates of impact basins with magnetic field strength to show that the previously established date of heavy bombardment, about 3.9 billion years ago, corresponds to the death of Mars's dynamo.

Could the bombardment have released enough energy to trigger the shutdown? Lillis, Manga, and planetary geophysicist James Roberts of John Hopkins University Applied Physics lab in Laurel, Maryland, have modeled the effects of heat produced by impacts. When they added the heat release from the biggest asteroids to models of mantle convection, they found that the mantle became a heating blanket rather than an ice pack. The extra heat was enough to stop core convection, the team reports in the current issue of the Journal of Geophysical Research – Planets.

Mars was hit by at least five particularly large asteroids during the bombardment. "Any one of the super-giant impacts could have shut off [the dynamo]," says Roberts. Earth likely suffered the same onslaught, but at twice the radius of Mars, it probably had a strong enough dynamo to withstand or recover from huge impacts.

Not all scientists are on board with the analysis. David Stevenson, a planetary scientist at the California Institute of Technology in Pasadena, suggests that although the explanation is plausible, he's not convinced the collisions released enough energy. Furthermore, "the dynamo does not need to have an external influence to stop functioning," he points out, adding that without enough core convection, "it may simply die of its own accord."